Sulfur oxide adsorbents and emissions control

ABSTRACT

High capacity sulfur oxide absorbents utilizing manganese-based octahedral molecular sieve (Mn—OMS) materials are disclosed. An emissions reduction system for a combustion exhaust includes a scrubber  24  containing these high capacity sulfur oxide absorbents located upstream from a NOX filter  26  or particulate trap.

GOVERNMENT RIGHTS

This invention was made with Government support under Contract NumberDE-AC06-76RLO1830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

TECHNICAL FIELD

The present invention is generally related to pollution control, andmore particularly, but not exclusively, is directed to high capacitysulfur oxide adsorbents and uses therefore in emissions control.

BACKGROUND

The combustion waste gases (i.e. the exhaust) of thermal power plants,factories, on-road vehicles, diesel generators, and the like containSO_(x) and NO_(x). State and federal regulations limit the permissibleamounts of these emissions because they create environment problems,such as acid rain. Accordingly, there is a continual need forimprovements in the cost effective and efficient control of theseemissions.

One mechanism for limiting NO_(x) and SO_(x) emissions is to remove orscrub the pollutants from the exhaust gas using an absorption bed, trapor similar device. Because many NO_(x) traps have been found to bepoisoned by the presence of SO_(x), it is important to remove as muchSO_(x) from the exhaust gas as possible. However, as compared to thelarge volume of studies on NO_(x) reduction, sulfur oxide removal usingsolid adsorbents is an area in need of scientific advancement. Forexample, certain types of materials have been identified as possiblesolid absorbents for use in a SO_(x) adsorption bed or traps, forexample calcium oxide and alkalized alumina (Na/Al₂O₃ or K/Al₂O₃),copper-based adsorbents, e.g. Cu/Al₂O₃, promoted metal oxides, e.g.TiO₂, Al₂O₃, ZrO₂, promoted cerium oxide (La- or Cu-doped CeO₂), andsupported cobalt (Co/Al₂O₃). Unfortunately, over the temperature rangeof about 250° C. to 475° C., these materials typically have a relativelylow absorption capacity. For example, their total adsorption capacity ofSO₂ is typically less than about 10 wt % based on the weight of theabsorbent, and their breakthrough absorption capacity can besubstantially lower, depending on operating conditions. As it iscombustion in this temperature range that leads to a significant portionof the total SO_(x) emissions, a greater adsorption capacity at thesetemperatures is needed.

One approach to increasing the absorption capacity of SO_(x) absorptionbeds is to provide an oxidation catalyst upstream or admixed with thebed so as to convert most of the SO₂ to SO₃, since SO₃ is generally morereadily adsorbed than SO₂ due to its formation of stable surfacesulfates. However, the cost of recovery of the oxidation catalyst(frequency a precious metal) and the relatively poor conversionefficiency of SO₂ to SO₃ at temperatures below about 300° C. limits theeffectiveness of this approach as well.

Accordingly, there is a need for solid SO_(x) absorbents with highabsorption capacity at lower temperatures and which reduce or eliminatethe need for separate oxidation catalysts. In one aspect the presentinvention addresses this need and provides a major improvement to SO_(x)absorption for emissions control.

SUMMARY

The present invention provides systems and techniques for SO_(x)emission control. While the actual nature of the invention coveredherein can only be determined with reference to the claims appendedhereto, certain aspects of the invention that are characteristic of theembodiments disclosed herein are described briefly as follows.

In one form, the invention concerns materials for absorbing, trapping,or otherwise eliminating oxides of sulfur from gases, for example thosesulfur oxides present in exhaust gases of internal combustion engines.The materials are mixed oxides having a framework of metal cations Meach surrounded by 6 oxygen atoms wherein the octahedra (MO₆) thusformed are connected together by edges and vertices generating astructure that produces channels in at least one direction in space. Thesides of these channels are formed by linking the octahedra (MO₆), whichconnect together by the edges, these sides connecting themselvestogether via the vertices of the octahedra. Thus, the width of thechannels can vary depending on whether the sides are composed of 2, 3and/or 4 octahedra (MO₆), which in turn depends on the mode ofpreparation. This type of material is known by its acronym OMS,Octahedral Molecular Sieve. In accordance with one form of theinvention, the materials are selected so that they have a structure thatgenerates channels either with a square cross section composed of, forexample, one octahedra by one octahedra (OMS 1×1), two octahedra by twooctahedra (OMS 2×2) or three octahedra by three octahedra (OMS 3×3), orwith a rectangular cross section composed of, for example, two octahedraby three octahedra (OMS 2×3). Thus, certain of the materials will have apyrolusite (OMS 1×1), hollandite (OMS 2×2), romanechite (OMS 2×3) ortodorokite (OMS 3×3) type structure. Other OMS structures, such as 1×3,1×4 or 2×4, 3×4 or 4×4, are also contemplated, though the 2×2 structurehas been found to be particularly effective in certain applications.

The OMS materials of the present invention are preferably manganesebased (Mn—OMS), which means that a major portion of the metal cation Mis manganese (Mn). An element is in the majority when it satisfies thefollowing formula: (n_(maj)/Σn_(M))>1/N, where n_(M) is the number ofatoms of element (M) and N is the number of different elements (M)composing the framework, n_(maj) being the highest number of atoms ofelement (M). Most preferably, over 50% of the elements M are manganesefor example at least 75% or at least 90% of the element M by mole.Preferably the manganese has an oxidation number between +2 and +4. Thebalance of element M can include one or more elements from groups IIIBto IIIA in the periodic table such as Zn²⁺, Co²⁺, Ni²⁺, Fe²⁺, Al³⁺,Ga³⁺, Fe³⁺, Ti³⁺, In³⁺,Cr³⁺, Si³⁺, Ge⁴⁺, Ti⁴⁺, Sn⁴⁺, and Sb⁵⁺ andcombinations thereof. The material used in this invention has acharacteristic structure of high surface area and may be capable ofoxidizing SO₂ to SO₃ and converting SO₃ to a sulfate. In certain cases,but not all cases, another cation, such as H⁺, NH₄ ⁺, Li⁺, Na⁺, Ag⁺, K⁺,Rb⁺, Tl⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ra²⁺, Cu²⁺, Pb²⁺, locates in thechannels in the OMS structures.

The absorbing phase of materials with type OMS 2*2, OMS 2*3 and OMS 3*3of one aspect of the invention has a three-dimensional structure thatgenerates channels in at least one direction in space, is composed ofoctahedra (MO₆) and comprises:

-   -   at least one element (M) selected from the group formed by        elements from groups IIIB, IVB, VB, VIB, VIIB, VIII, IB, IIB,        IIIA of the periodic table and germanium, each element M being        coordinated with 6 oxygen atoms, and located at the center of        the oxygen octahedra, wherein a major portion of M is manganese;        and    -   at least one element (B) selected from the group formed by the        alkali elements IA, the alkaline-earth elements IIA, the rare        earths IIIB, transition metals or elements from groups IIIA and        IVA, element B generally being located in channels in the oxide        structure.

More particularly, elements M are selected from scandium, titanium,zirconium, vanadium, niobium, chromium, molybdenum, tungsten, manganese,iron, cobalt, nickel, copper, zinc, aluminum, gallium, and mixturesthereof.

The average charge (oxidation number) carried by the cation or cations Mfrom groups IIIB to IIIA is preferably about +3.5 to +4. Preferably, atleast about 50% of the elements (M) in the material are manganese,titanium, chromium, aluminum, zinc, copper, zirconium, iron, cobalt,and/or nickel. More preferably, over 50% of the elements (M) aremanganese, chromium, copper, iron, titanium and/or zirconium. In oneform, manganese composes at least about 50% of the M element by mole,for example at least 75% or 90% of the M element.

Other elements M from groups IIIB to IIIA can be added in minorquantities as dopants. Preferably, the elements from groups IIIB to IIIAadded in minor quantities are selected from scandium, titanium,zirconium, vanadium, niobium, chromium, molybdenum, tungsten, manganese,iron, cobalt, nickel, copper, zinc, aluminum, gallium, and mixturesthereof.

Elements (B) belong to the group formed by the alkali elements IA,alkaline-earth elements IIA, rare earth elements IIIB, transition metalsand elements from groups IIIA and IVA. They are located in the channelsof the material. Preferably, metal B is selected from the group formedby potassium, sodium, magnesium, barium, strontium, iron, copper, zinc,aluminum, rubidium and calcium and mixtures thereof.

A number of different methods exist for preparing these materials (seereferences 3 and 4 below, for example). They may be synthesized bymixing and grinding solid inorganic precursors of metal oxides (metals Mand B), followed by calcining. The materials can also be obtained byheating solutions of precursor salts to reflux, drying and calcining, byprecipitating precursor salts by the sol-gel method, or by hydrothermalsynthesis which consists of heating an aqueous solution containing theelements constituting the final material under autogenous pressure. Thematerials obtained from these syntheses can be modified by ion exchangeor isomorphous substitution.

Optional metal (C) is introduced using any of the methods known to theskilled person: excess impregnation, dry impregnation, ion exchange,etc.

The material of the invention generally has a specific surface area inthe range 1 to 300 m²/g, preferably in the range 2 to 300 m²/g, and morepreferably in the range 30 to 250 m²/g. The adsorption kinetics arebetter when the specific surface area is high, i.e., in the range 10m²/g such as in the range 30 to 250 m²/g.

The sorbent phase can be in the form of a powder, beads, pellets orextrudates; they can also be deposited or directly prepared onmonolithic supports of ceramic or metal. To increase the dispersion ofthe materials and thus to increase their absorption capacity, thematerials can be deposited on porous supports with a high specificsurface area before being formed (extrusion, coating . . . ). Thesesupports are generally selected from the group formed by the followingcompounds: alumina (alpha, beta, delta, gamma, khi, or theta alumina),silicas (SiO₂), silica-aluminas, zeolites, titanium oxide (TiO₂),zirconium oxide (ZrO₂), magnesium oxide (MgO), divided carbides, forexample silicon carbides (SiC), used alone or as a mixture. Mixed oxidesor solid solutions comprising at least two of the above oxides can beadded.

For many uses, such as in connection with a vehicle exhaust, it isusually preferable to use rigid supports (monoliths) with a large openporosity (more than 70%) to limit pressure drops that may cause high gasflow rates, and in particular high exhaust gas space velocities. Thesepressure drops are deleterious to proper functioning of the engine andcontribute to reducing the efficiency of an internal combustion engine(gasoline or diesel). Further, the exhaust system is subjected tovibrations and to substantial mechanical and thermal shocks, socatalysts in the form of beads, pellets or extrudates run the risk ofdeterioration due to wear or fracturing.

Two techniques are used to prepare the catalysts of the invention onmonolithic ceramic or metal supports (or substrates).

The first technique comprises direct deposition on the monolithicsupport, using a wash coating technique which is known to the skilledperson, to coat the adsorbing phase prepared using the operatingprocedure described, for example, in reference (4) below. (S. L. Suib,C-L O'Young, “Synthesis of Porous Materials”, M. L. Occelli, H. Kessler,eds, M. Dekker, Inc., p. 215, 1997). The adsorbent phase can be coatedjust after the co-precipitation step, hydrothermal synthesis step orheating under reflux step, the final calcining step being carried out onthe phase deposited on the monolith, or the monolith can be coated afterthe material has been prepared in its final state, i.e., after the finalcalcining step.

The second technique comprises depositing the inorganic oxide on themonolithic support and then calcining the monolith between 500° C. and1100° C. so that the specific surface area of this oxide is in the range20 to 150 m²/g, then coating the monolithic substrate covered with theinorganic oxide with the adsorbent phase obtained after the stepsdescribed in the reference (4).

Monolithic supports that can be used include: ceramics, where theprincipal elements can be alumina, zirconia, cordierite, mullite,silica, alumino-silicates or a combination of several of thesecompounds; a silicon carbide and/or nitride; an aluminium titanate;and/or a metal, generally obtained from iron, chromium or aluminiumalloys optionally doped with nickel, cobalt, cerium or yttrium.

The structure of a ceramic supports can be that of a honeycomb, or theyare in the form of a foam or fibers.

Metal supports can be produced by winding corrugated strips or bystacking corrugated sheets to constitute a honeycomb structure withstraight or zigzag channels which may or may not communicate with eachother. They can also be produced from metal fibers or wires which areinterlocked, woven or braided.

With supports of metal comprising aluminum in their composition, it isrecommended that they are pre-treated at high temperature (for examplebetween 700° C. and 1100° C.) to develop a micro-layer of refractoryalumina on the surface. This superficial micro-layer, with a porosityand specific surface area which is higher than that of the originalmetal, encourages adhesion of the active phase and protects theremainder of the support against corrosion.

The quantity of sorbent phase deposited or prepared directly on aceramic or metallic support (or substrate) is generally in the range 20to 300 g per liter of said support, advantageously in the range 50 to200 g per liter.

The materials of the invention can thus adsorb oxides of sulfur presentin the gases, in particular exhaust gases. These materials are capableof adsorbing SO_(x) at a temperature which is generally in the range 50°C. to 650° C., preferably in the range 100° C. to 600° C., morepreferably in the range 150° C. to 550° C.

For diesel engines in automobiles, an intended application, thetemperature of the exhaust gas may be in the range 150° C. to 500° C.and rarely exceeds 600° C. The materials used in the process of theinvention are thus suitable for sorbing oxides of sulfur present in theexhaust gases of stationary engines or, particularly, automotive dieselengines or spark ignition (lean burn) engines, but also in the gasesfrom gas turbines operating with gas or liquid fuels. These exhaustgases typically contain oxides of sulfur in the range of a few tens to afew thousands of parts per million (ppm) and can contain comparableamounts of reducing compounds (CO, H₂, hydrocarbons) and nitrogenoxides. These exhaust gases might also contain larger quantities ofoxygen (1% to close to 20% by volume) and steam, though the presentsorbents can be effective in oxygen free environments as well. Thesorbent material of the invention can be used with HSVs (hourly spacevelocity, corresponding to the ratio of the volume of the monolith tothe gas flow rate) for the exhaust gas generally in the range 500 to150,000 h⁻¹, for example in the range 5,000 to 100,000 h⁻¹.

In has been found that absorption of SO_(x) leads to a noticeable colorchange in the sorbent. Accordingly, in one variation, a SO_(x) trap isprovided by a quantity of the sorbent contained in a housing having awindow. The color of the sorbent material is periodically monitoredthrough the window with the need to replace or recharge the trapindicated by the color change. In this or other refinements, a spentSO_(x) trap can be regenerated by appropriate reflux synthesis so as toreuse the sorbent support and the housing.

The SO_(x) sorbent material of the present invention can be usedanywhere SO_(x) needs to be absorbed. In has been found that significantadvantages can be realized in the overall control of emissions from acombustion exhaust by locating the SO_(x) sorbent material upstream froma particulate filter or NO_(x) trap.

According to another aspect of the invention, SO_(x) adsorption isprovided by a manganese oxide material which has inherent oxidizingcapability, so that SO₂ can be oxidized and absorbed without use of aseparate and costly oxidation catalyst, and which has a high totalabsorption capacity, for example greater than about 40% by weight,thereby providing economical and efficient emissions control.

BRIEF DESCRIPTION OF THE FIGURES

Although the characteristic features of this invention will beparticularly pointed out in the claims, the invention itself, and themanner in which it may be made and used, may be better understood byreferring to the following description taken in connection with theaccompanying figures forming a part thereof.

FIG. 1 is exemplary plots of the absorption of SO₂ on 2×2 Mn—OMSmaterials and on MnO₂.

FIGS. 2 a and 2 b are exemplary scanning electron microscopy images of a2×2 Mn—OMS material before and after SO₂ absorption, respectively.

FIGS. 3 a and 3 b are exemplary x-ray diffraction patterns of a 2×2Mn—OMS material before and after SO₂ adsorption, respectively.

FIG. 4 is exemplary plots of absorption of SO₂ on an Mn—OMS material atdifferent gas feed temperatures.

FIG. 5 is exemplary plots of adsorption of SO₂ on an Mn—OMS material atdifferent gas feed rates.

FIG. 6 is exemplary plots of adsorption of SO₂ on an Mn—OMS material atdifferent concentrations of SO₂ in the feed gas.

FIG. 7 is exemplary plots of absorption of SO₂ on an Mn—OMS material atdifferent feed gas compositions.

FIG. 8 is exemplary plots of absorption of SO₂ on 2×3 Mn—OMS materialsat different gas feed rates.

FIG. 9 is exemplary plots of absorption of SO₂ on a 2×4 Mn—OMS material.

FIG. 10 is a schematic illustration of an emissions control systemimplemented on a vehicle producing combustion exhaust according to anembodiment of the invention.

FIG. 11 is a perspective view of a SO_(x) filter having a monitoringwindow and regeneration ports according to an embodiment of theinvention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is hereby intended. Alterations and further modifications inthe illustrated devices, and such further applications of the principlesof the invention as illustrated herein are contemplated as wouldnormally occur to one skilled in the art to which the invention relates.

In one form, the present invention employs a manganese based octahedralmolecular sieve as a high capacity sulfur oxide solid absorbent. Ascompared to other adsorbents studied for the removal of SO₂ from wastegases, this material provides surprising high capacity and efficiency.In a preferred form, this material is referred to as Mn—OMS 2×2.

The basic structure of the materials employed in the Examples thatfollow consists of MnO₆ octahedra joined at edges to form a 2×2hollandite tunnel structure with a pore size of about 0.46 nm. (1) Forcryptomelane, a counter-cation, K⁺, is present within the tunnelstructure for charge compensation. Mn can assume an oxidation state of4+, 3+, or 2+, and the average Mn oxidation state can be controlledwithin a certain range during synthesis. Generally, this material has ahigh surface area (˜80 m²/g), and high redox reaction activity. (2)

Without intending to be bound by any theory of operation, the presentinvention is based on carrying out the following reaction:SO₂+K_(x)Mn₈O₁₆→MnSO₄+K₂O  (1)SO₂ is oxidized to SO₃ by Mn⁴⁺ and Mn³⁺, and Mn⁴⁺ and Mn³⁺ aresimultaneously reduced to Mn²⁺ (MnO). The SO₃ produced then reacts withMn²⁺ to form MnSO₄.

As explained herein, tunnel structure cryptomelane was found to be ahigh capacity sulfur dioxide adsorbent. Its SO₂ capacity from 250° C. to475° C. is more than ten times higher than that of conventional SO₂adsorbents. Its maximum SO₂ capacity can be as high as about 74 wt %.The dominant mechanism for SO₂ absorption is believed to be that SO₂ isoxidized by Mn⁴⁺ and Mn³⁺ to SO₃, with the SO₃ reacting with theco-produced Mn²⁺ to form MnSO₄. It has been found that this reaction isprimarily controlled by the mass diffusion of SO₂ through the adsorbent,and that it can surprisingly effectively occur in an oxygen-freeenvironment. In addition, the visibly significant color change ofcryptomelane from black to yellow after SO₂ absorption can be used as aconvenient indicator for the adsorbent replacement.

Cryptomelane for SO₂ absorption can be synthesized either from a mixtureof KMnO₄ and MnSO₄ or a mixture of MnSO₄ and KOH solution. After SO₂absorption, MnSO₄ is formed, which can subsequently be dissolved inwater and used as raw material for a subsequent cryptomelane synthesis.To regenerate the SO₂ absorption trap, therefore, only KOH and O₂ isneeded because the adsorbent support (such as a monolith) and the MnSO₄can be re-used.

This highly efficient SO₂ adsorbent can be used for removal of SO₂generated from thermal power plants, factories, and on-road vehicles. Itcan be especially effective for removal of SO_(x) that is present in theemissions of diesel trucks, in order to protect downstream emissionscontrol devices such as particulate filters and NOx traps that arepoisoned by SO_(x).

Turning now to FIG. 10, a vehicle 20 implementing a simplified emissionscontrol system according to the present invention is depicted. Vehicle20 has an engine 22 fluidly connected to upstream and a downstreamemissions control devices 24 and 26 respectively. Devices 24 and 26perform different emissions control functions, and while they could becombined into a single device, as described more fully below, certainproblems are avoided by the provision of separate devices.

The exhaust 21 from the engine 22 is first fed to the upstream device24. The transfer of the exhaust, and all other fluid transfer operationcan be in any conventional fashion, such as the exhaust piping of aconventional automobile, and may include intermediate fluid processingoperations, such as catalytic conversion, mixing with other gases, orrecycling of exhaust to the engine.

The upstream device 24 is a SO_(x) scrubber, whose function is to removeany sulfur oxides from the exhaust gas 21 and to prevent their passagevia channel 25 to the downstream device 26 and eventually the exhaust 28to the atmosphere. The SO_(x) scrubber functions to remove most if notall of any sulfur oxides in the gaseous exhaust 21. The SO_(x) scrubbercontains a solid SO_(x) sorbent as described herein, preferably onesupported on a monolith or similar support. The sorbent removes theSO_(x) from the passing gas stream, for example by permanent orreversible sorption (adsorption or absorption) trapping, filtering, orchemical reaction therewith, and thereby prevents SO_(x) from enteringthe downstream device 26.

The downstream device 26 provides a different emissions control functionthan the upstream device 24. As illustrated, the downstream device is aNO_(x) scrubber or particulate filter and any conventional scrubber orfilter can be employed. As many conventional NO_(x) traps and/orparticulate filters are fouled or poised by the presence of SO_(x), theprovision of upstream device 24 inventively reduces or eliminates thispossibility by providing an inlet stream to device 26 that issubstantially SO_(x) free. For example, it is contemplated that device24 will function to cause fluid at 25 to have less than 1% of the SO_(x)concentration in the exhaust 21, more preferably less than about 0.1%.

Turning now to FIG. 11, an exemplary SO_(x) scrubber 30, which can beemployed for device 24 in the FIG. 10 system, is depicted. Scrubber 30includes a housing 32 having a fluid inlet 34, a fluid outlet 36 and afluid flow path therebetween. The housing contains a SO_(x) sorbent inthe flow path so as to facilitate the removal of SO_(x) from the fluidas it passes through the SO_(x) scrubber. Housing 32 also contains awindow 38 providing visual access to the sorbent contained therein. Asthe sorbent contained in the housing 32 absorbs the SO_(x), it willundergo a noticeable color change, with the sorbent nearer the inlet 34becoming saturated (and thus changing color) sooner than the sorbentnear the outlet 36. The resulting transition between different coloredportions of the sorbent provides an indication on the extent that thesorbent packing has becoming spent. Accordingly, a series of indicatormarks 39 are provided on the window 38 or on the housing 32 adjacent thewindow 38 for measuring the remaining sorption capacity of the scrubber30. For example, during routine maintenance of a machine on whichscrubber 30 is implemented, vehicle 20 for example, the window 38 can bechecked to determine whether replacement of the scrubber 30 isnecessary.

When replacement is needed, i.e. the sorbent is saturated and entirelychanged colors, the scrubber 30 can simply be removed from the exhauststream and replaced. In another form of the invention, once removed, thespent sorbent and/or the scrubber 30 can be reused. For example when thespent sorbent is converted to MnSO₄, this MnSO₄ can be used as astarting material to reform the Mn—OMS material on the support. Thisreforming can be accomplished by removing the spent sorbent and itssupport (such as a monolith) from the housing 32. After processing andappropriate calcination the spent sorbent is returned to its OMSstructure and is ready to absorb additional SO_(x).

Alternatively, the necessary reagents for reforming the spent sorbent,for example KOH and O₂, can be circulated through the housing 32 withoutremoving the spent sorbent. The inlet and outlet ports 34 and 36 can beused as the reagent inlet and outlet ports to recharge the sorbent inthis fashion when the scrubber 30 is removed from the exhaust stream.However, as illustrated, scrubber 30 includes optional dedicated inletand outlet ports 44 and 42 for this purpose. Ports 42, 44 permitrecharging without removal from the exhaust, or they may be used inconjunction with offline recharging via inlet and outlet 34, 36.

Reference will now be made to examples illustrating specific features ofinventive embodiments. It is to be understood, however, that theseexamples are provided for illustration and that no limitation to thescope of the invention is intended thereby. Further, certainobservations, hypotheses, and theories of operation are presented inlight of these examples in order to further understanding, but these arelikewise not intended to limit the scope of the invention.

EXAMPLES Example 1 Sample Preparations and Test Conditions OMS 2×2

2×2 manganese based octahedral molecular sieve (tunnel structurecryptomelane) was prepared using the methods developed by DeGuzman, etal. (3) A typical synthesis was carried out as follows: 11.78 g KMnO₄ in200 ml of water was added to a solution of 23.2 g MnSO₄.4H₂O in 60 ml ofwater and 6 ml of concentrated HNO₃. The solution was refluxed at 100°C. for 24 h, and the product was washed and dried at 120° C.Hydrothermal reaction in Teflon bottles at 90° C., instead of the refluxmethod, was also used for the synthesis.

An alternative synthesis method for cryptomelane was purging O₂ throughmixture of MnSO₄ and KOH solution, followed by calcination at 600° C.(3) A typical preparation was: a solution of 15.7 g KOH in cold 100 mlof water was added to a solution of 14.9 g of MnSO₄.H₂O in 100 ml ofwater. Oxygen gas was bubbled (about 10 L/min) through the solution for4 hours. The product was washed with water and calcined in air for 20 h.

The dried materials were sieved to provide 40–80 mesh particles for theSO₂ absorption tests, which was carried out in a temperature controlledreactor with a Sulfur Chemiluminescent Detector (SCD) analysis system.Unless otherwise stated, the absorption testing conditions were 0.5 gram40–80 mesh absorbent particles, 100 standard cubic centimeters perminute (sccm) exhaust air flow with 250 parts per million (ppm) SO₂, 75%N₂, 12% O₂, and 13% CO₂.

The SO₂ absorption performance of cryptomelane material synthesized byrefluxing mixture of KMnO₄ and MnSO₄ solutions was also systemicallytested under different temperature, gas hour space velocity (GHSV), SO₂concentrations, and feed gas compositions, and the results aresummarized in Table 1.

Before each SO₂ absorption measurement, the material was heated at 500°C. for 2 h in flowing air. To characterize the property changes beforeand after SO₂ absorption, powder X-ray diffraction pattern (XRD),particle surface area (SA), and scanning electron microscopy (SEM)images were collected on some of the tested materials.

TABLE 1 SO₂ absorption test conditions for cryptomelane materialVariable conditions Other conditions SO₂ absorption temperature 0.5 g40–80 mesh absorbent, feed 250° C., 325° C., gas: 250 ppm SO₂, 82% N₂,18% O₂, and 475° C. ~8,000 hr⁻¹ GHSV Gas Hour Space Velocity, GHSV, hr⁻¹0.5 g 40–80 mesh absorbent for 8K  8000, and 30K hr⁻¹ GHSV test, and0.25 g 30000, for 60K test, and 60000 325° C., feed gas: 250 ppm SO₂,82% N₂, 18% O_(2.) SO₂ Concentration in feed gas 0.5 g 40–80 meshabsorbent for 250 50 ppm ppm SO₂ test, and 0.25 g for 50 ppm and 250 ppmtest 325° C., feed gas: 82% N₂, 18% O₂, ~30000 hr⁻¹ GHSV, Feed gascomposition Air For CO—NO—H₂O test (250 ppm SO₂, 82% N₂ and 18% O₂) 0.25g 40–80 mesh adsorbent, 325° C., CO₂ effect 17K hr⁻¹ GHSV (250 ppm SO₂,75% N₂, 12% O₂, and 13% CO₂) For others NO effect 0.5 g 40–80 meshadsorbent, 325° C., (178 ppm SO₂, 178 ppm NO, 9% N₂, 20% O₂, 71% He) ~8Khr⁻¹ GHSV CO effect (250 ppm SO2, 250 ppm CO, 87% N₂, 13% O₂) CO—NO—H₂Oeffect (125 ppm SO₂, 125 ppm CO, 125 ppm NO, 11% H₂O*, 19% N₂, 20% O₂,50% He) O₂-free effect (250 ppm SO₂, 12.5% N₂, and 87.5% He) *Steam wasintroduced by purging O₂ through flask containing temperature-controlledde-ionized water. After passing through the absorbent, steam was removedbefore SCD detector using MD Gas Dryer (from Perma Pure Inc.) which canselectively separate H₂O from gases mixture.

Example 2 (Comparative) Comparative Breakthrough Absorption Capacities

For purposes of comparison, SO₂ absorption capability of severalcommercially available materials were tested at a temperature range from250° C. to 475° C. under the testing conditions indicated above (0.5gram 40–80 mesh absorbent particles, 100 sccm exhaust air flow with 250ppm SO₂, 75% N₂, 12% O₂, and 13% CO₂). These materials included La₂O₃ orBaO doped ZrO₂—CeO₂ mixtures (from Daiichi Kigenso Kagaku Kogyo Co.,Ltd.), ZrO₂ (from RC100, Inc.), Al₂O₃ (from Engelhard, acidic), CaO(from Alfa Aesar Inc), and MnO₂ (from Erachem Comilog, Inc.) which wereobtained from their respective commercial sources.

Table 2 presents a summary of the SO₂ absorption capacities for certainof these SO₂ adsorbents. The SO₂ capacity was calculated based on weightof SO₂ adsorbed per gram of adsorbent when 1% of the initial SO₂concentration was observed eluting from the absorbent bed. This isdefined as the breakthrough absorption capacity. As seen in Table 2, theSO₂ breakthrough absorption capacities for these materials are generallyless than 5 wt %. Of the other materials tested, the absorptioncapacities for the MnO₂ was selected for more direct comparison to thematerials of Example 1 and are presented in the Examples below.

TABLE 2 SO₂ breakthrough absorption capacity of conventional SO₂adsorbents Materials tested 200° C. 325° C. 400° C. 475° C. 73.8% ZrO₂−26.2% CeO₂ mixed oxide 2.2 wt % 2.2 wt % 2.2 wt % SA 53.5 m²/g, 10000hr⁻¹GHSV 73.2% ZrO₂, 1.75% La₂O₃, 5.22% Nd₂O₃, and 2.4 wt % 3.1 wt % 3.6wt % 19.9% CeO₂ mixed oxide SA 60.3 m²/g, 7236 hr⁻¹GHSV 61.8% ZrO₂,29.4% CeO₂, and 8.9% La₂O₃ mixed 2.0 wt % 3.5 wt % 5.0 wt % 5.3 wt %oxide SA 69.1 m²/g, 11400 hr⁻¹GHSV 70.3% ZrO₂, 4.0% BaO, and 25.8% CeO₂mixed 1.7 wt % 1.7 wt % 2.5 wt % oxide SA 29.2 m²/g, ~10000 hr⁻¹GHSVZrO₂, SA 95.7 m²/g, 2.2 wt % 10000 hr⁻¹GHSV Al₂O₃, Engelhard Corp. SA150 m²/g, Acidic 1.0 wt % 7,281 hr⁻¹ GHSV CaO, SA 2.7 m²/g, ~10,000hr⁻¹GHSV <0.2 wt %   <0.2 wt %   <0.2 wt %   1. Other test conditions:0.5 g 40–80 mesh absorbent, feed gas: 250 ppm SO₂, 75% N₂, 12% O₂, and13% CO₂ 2. SO₂ capacity based on gram of SO₂ adsorbed per gram ofcatalyst at 1% SO₂ breakthrough point

Example 3 Breakthrough Absorption Capacities

Table 3 gives the SO₂ breakthrough and total absorption capacities ofthe materials synthesized according to Example 1, K_(x)Mn₈O₁₆ A (refluxsynthesis, with projected final average Mn oxidation state 3.5⁺),K_(x)Mn₈O₁₆ B (reflux synthesis, with projected final average Mnoxidation state 4⁺), Cu-doped K_(x)Mn₈O₁₆ B (hydrothermal synthesis,with projected final average Mn oxidation state 4⁺), and K_(x)Mn₈O₁₆ C(synthesized from MnSO₄ and KOH). The projected final Mn oxidation state(PAOS) is calculated based on the relative amount of KMnO₄ and MnSO₄ inthe starting solution, i.e. PAOS=(moles of KMnO₄*7+moles ofMnSO₄*2)/(moles of KMnO₄+moles of MnSO₄). Breakthrough capacities weremeasured at 1% breakthrough as described in Example 2, and total SO₂absorption capacity was also measured with the values given inparentheses in Table 3. For example, the breakthrough and maximum SO₂absorption capacities for K_(x)Mn₈O₁₆ B are 58 wt % and 68 wt %respectively. Under similar reaction conditions, these materials havesignificantly higher breakthrough SO₂ absorption capacity than theconventional SO₂ adsorbents given in Table 2. To facilitate comparison,the results for the commercially obtained electrolytic MnO₂ (EMD,Erachem Comilog, Inc.) discussed above are presented in Table 3.

TABLE 3 SO₂ absorption capacity of cryptomelane materials synthesizedSO₂ breakthrough SA, capacity Materials tested m²/g (total capacity)K_(x)Mn₈O₁₆ A, 14000 hr⁻¹ GHSV 51   28 (45)^(b) wt % Cu-dopedK_(x)Mn₈O₁₆ B^(a)) 9637 hr⁻¹ GHSV 88 57.5 (67) wt % K_(x)Mn₈O₁₆ B, ~7500hr⁻¹ GHSV 74   58 (68) wt % K_(x)Mn₈O₁₆ C, ~8000 hr⁻¹ GHSV 32   48 (xx)wt % EMD MnO₂, 12000 hr⁻¹ GHSV 30  3.5 (9) wt % *Other test conditions:0.5 g 40–80 mesh absorbent, 325° C., feed gas: 250 ppm SO₂, 82% N₂, and18% O₂ for K_(x)Mn₈O₁₆ C, for others: 75% N₂, 12% O₂, and 13% CO₂^(a)CuSO₄ was added in MnSO₄ solution. ^(b)Data in parentheses aremaximum SO₂ absorption capacities.

Example 4 SO₂ Absorption at 325° C.

FIG. 1 is a plots showing SO₂ absorption on K_(x)Mn₈O₁₆ A, B, Cu-dopedK_(x)Mn₈O₁₆ B, and EMD MnO₂, as a function of the weight percentage ofSO₂ fed at 325° C. The left axis is the percentage of SO₂ not absorbed(i.e. that passed through the bed) and corresponds to the S-shapedcurves. The right axis is the wt % SO₂ absorbed and corresponds to thecurves whose slope is initially 1 at low feed amounts and then tendstowards slope of zero at high feed amounts. All weight percentages arerelative weight of absorbent.

Example 5 Changes After Absorption

FIGS. 2 and 3 are before and after Scanning Electron Microscopy (SEM)Images and x-ray diffraction patterns (XRD), respectively, for the SO₂absorption by K_(x)Mn₈O₁₆ B at 325° C. As shown in FIG. 2, themorphology and the crystal structure of the K_(x)Mn₈O₁₆ materialsignificantly changes after SO2 absorption. The surface area of thismaterial also decreased sharply from 74 m²/g to 4.6 m²/g, and the XRDpatterns indicate that the OMS structure had converted to a mixture ofMnSO₄ and manganolangbeinite K₂Mn₂(SO₄)₃. A visible color change in theabsorbent was also evident. It was initially black and changed to yellowafter the SO₂ absorption.

Example 6 Temperature Dependence

FIG. 5 is a plot of the wt % of SO₂ absorption on K_(x)Mn₈O₁₆ B at 250°C., 325° C. and 475° C. under the other test conditions as indicated inTable 1 (0.5 g 40–80 mesh absorbent, feed gas: 250 ppm SO₂, 82% N₂, 18%O₂,˜8,000 hr⁻¹ GHSV). Even at as low a temperature as 250° C., thismaterial could adsorb more than 66 wt % SO₂, although absorption is not100% and some SO₂ breakthrough was observed even initially.

Example 7 Feed Gas Flow Rate Dependence

FIG. 6 shows the feed gas GHSV effect on the SO₂ absorption onK_(x)Mn₈O₁₆ B at 325° C. The breakthrough SO₂ capacity decreased from61, to 44, and 33 wt % as the feed GHSV increased from 8K, to 30K and60K hr⁻¹. As the feed GHSV increased, the total SO₂ absorption capacitydecreased, but not significantly, from 74 to 64 and 63 wt %.

Example 8 Feed Gas Composition Dependence

FIG. 7 shows that increasing SO₂ concentration in the feed gas from 50ppm to 250 ppm almost had no effect on the SO₂ adsorption on K_(x)Mn₈O₁₆B. FIG. 8 shows feed gas composition effect on the SO₂ absorption onK_(x)Mn₈O₁₆ B at 325° C. CO and NO, which also exist in the combustionwaste gases, did not have any effect on the SO₂ absorption onK_(x)Mn₈O₁₆. CO₂, at ˜13% level, could slightly, if any, decrease theSO₂ breakthrough capacity (from 61 wt % to 59 wt %) and the total SO₂capacity (from 74 to 68 wt %).

Surprisingly, the OMS material K_(x)Mn₈O₁₆ B shows a higher SO₂breakthrough capacity of 74 wt % and total SO₂ capacity of ˜80 wt % inCO—NO—SO₂—H₂O mixture feed gas even through the GHSV was 17K hr⁻¹. Whensteam was introduced into the system, the SCD signals were not verystable which may generate some error of the measurement. In the absentof O₂, cryptomelane material K_(x)Mn₈O₁₆ B still has SO₂ breakthroughcapacity of 41 wt % and total SO₂ capacity of 58 wt %. After SO₂absorption test, the weight gain of the adsorbent was measured though itwas not possible to collect all the adsorbent particles. Table 4 givesthe weight gain data of the feed gas composition effect tests. Forcomparison, the total SO₂ absorption calculated from SO₂ concentrationchange for each test was also listed.

TABLE 4 Adsorbent weight gain after SO₂ absorption test in differentfeed gas at 325° C. Feed Gas CO₂ CO NO O₂-free CO—NO—H₂O Air effecteffect effect effect effect Weight 65.7 65.6 66.2 64 50 66.8 Gain, %Total SO₂ 74.6 68.1 74.1 70.7 58 80.6 absorbed, % *See Table 1 fordetailed test conditionsDiscussion of Examples 1–8:

Based on reaction (1), the maximum SO₂ capacity is believed to becontrolled by oxidation state of Mn in the adsorbent. For the OMSmaterials K_(x)Mn₈O₁₆ B and Cu-doped K_(x)Mn₈O₁₆ B, the averageprojected oxidation state of Mn is 4; there are very few K⁺ cationspresent in the structure. If the small amount of K⁺ is ignored, themaximum SO₂ capacity should be ˜73.5 wt %. The measured maximum SO₂capacities (see Tables 3 and 4) are ˜70 wt % for these two materials,which is very close to 73.5 wt % and reflects the contribution of theK₂O byproduct. This result supports the hypothesis that reaction (1)dominates SO₂ absorption. Direct evidence is also seen in the XRDpatterns before and after the SO₂ absorption. After SO₂ absorption, theOMS structure completely changed to MnSO₄ and manganolangbeiniteK₂Mn₂(SO₄)₃. The formation of K₂Mn₂(SO₄)₃ means more SO₄ ²⁻ than Mn²⁺ isformed and that reaction (1) needs to be modified slightly. A postulateis that the cryptomelane material itself or the SO₂ adsorbed material(mostly MnSO₄) acts as a catalyst for the following reaction andformation of K₂Mn₂(SO₄)₃.SO₂+O₂→SO₃  (2)Because the amount of K⁺ is small, the whole SO₂ absorption process ismostly dominated by manganese oxidation. It should be noted that, whilethe presence of O₂ in the feed gas does help increase absorption, it isnot necessary, as demonstrated by the satisfactory absorptionperformance in the oxygen-free feed gas test (feed gas composition: 250ppm SO₂, 12.5% N₂, and 87.5% He). After SO₂ absorption in an O₂-freeenvironment, both MnSO₄ and K₂Mn₂(SO₄)₃ were formed, suggesting thatother reactions involving oxygen transfer and formation of someamorphous phases also happened in an O₂-free environment.

The synthesized K_(x)Mn₈O₁₆ A had an average projected Mn oxidationstate of +3.5. The ideal formula for this material should be K₄Mn₈O₁₆(after water removal at 500° C. and assuming all the counter-cations areK⁺). Then according to reaction (1), the maximum capacity is 45 wt %,which exactly matches the measurement (see Table 3). If reaction (2)also exists with this material, K₂Mn₂(SO₄)₃ should form, which was notseen in an XRD pattern (not shown), and the maximum SO₂ capacity shouldbe 60 wt %. This suggests that reaction (1) predominates, and thatreaction (2) occurs only to a very small extent or not at all. Based onthe results from K_(x)Mn₈O₁₆ A (hydrothermal synthesis, with projectedfinal average Mn oxidation state 3.5), K_(x)Mn₈O₁₆ B (reflux synthesis,with projected final average Mn oxidation state 4) and Cu-dopedK_(x)Mn₈O₁₆ B (hydrothermal synthesis, with projected final average Mnoxidation state 4), neither the choice between reflux or hydrothermalsynthesis, nor doping with Cu significantly changes SO₂ break throughabsorption capacity or maximum absorption capacity. This suggests thatthere is no kinetic or thermodynamic effect attributable to doping orthe synthesis mechanism.

The alternative synthesis method of purging O₂ through a mixture ofMnSO₄ and KOH solution followed by calcination at 600° C. did show someeffect. K_(x)Mn₈O₁₆ C, synthesized using this method, gives ˜50 wt % SO₂breakthrough capacity and ˜60 wt % SO₂ total capacity, which are lowerthan those for K_(x)Mn₈O₁₆ B. Potential reasons for this disparityinclude 1) possible incomplete oxidation of Mn²⁺ to Mn⁴⁺, 2) therelative low surface area (32 m²/g vs. 75 m₂/g) and high density (˜1g/cm³ vs. 0.67 g/cm³), and 3) the product is not pure OMS as a smallamount of K₂SO₄ was found to exists (see FIG. 9 for its XRD pattern).While the synthesis conditions could still be optimized to improveadsorption performance, the absorption performance is adequate to beeffective and consideration of other factors renders this a desirableapproach. For example, this synthesis method can significantly decreasethe overall cost of the absorbent production. After SO₂ absorption,almost pure MnSO₄ is formed, which, being one of the starting materials,can be recaptured and reused. Also the density of K_(x)Mn₈O₁₆ C is ˜50%higher than that of K_(x)Mn₈O₁₆ B, which indicates more adsorbents canbe loaded and higher SO₂ capacity in a given volume can be achieved.

Although electrolytic manganese dioxide (EMD) from Erachem Comilog, Inc.has Mn⁴⁺ and a surface area of ˜30 m²/g, the SO₂ capacity for thismaterial does not approach that of the OMS materials. This indicatesthat the OMS structure is important for high SO₂ absorption.

K_(x)Mn₈O₁₆ B was tested at a temperature 250° C., 325° C., and 475° C.At 250° C., a lower SO₂ absorption rate was observed but the maximum SO₂capacity is almost the same as that measured at 325° C. and 475° C. Incomparing the results at 325° C. and 475° C., minimal difference wasobserved except that at 475° C., after the breakthrough capacity wasreached, the SCD detector background slightly increased, indicating thatsome SO₃ was being released even though total absorption amount wasstill increasing.

Substantial variation in the feed gas flow rate affected the SO₂absorption performance of K_(x)Mn₈O₁₆ B adsorbent, though in practicethis affect can be mitigated with appropriate sizing and design of theSO_(x) trap. For example, the SO₂ breakthrough capacity decreased about50% when the feed GHSV increased from 8K to 60K hr⁻¹. This indicates theSO₂ absorption reaction is mostly controlled by SO₂ mass diffusionthrough the adsorbent. Since the SO₂ concentration in the feed gas hadlittle or no effect on its absorption suggests that reaction (1) is0^(th) order for SO₂, and it is mostly controlled by the availableactive sites on the adsorbent.

Most components in the simulated exhaust combustion gases tested, CO,NO, CO₂, and H₂O, did not have a significant effect on the SO₂absorption capacity of the K_(x)Mn₈O₁₆ B adsorbent, and the absorptioncapacity in an oxygen free environment, while lower, was stillacceptable. Therefore, it is expected that the Mn—OMS materials shouldbe useful to remove SO₂ from gas streams in a wide variety ofapplications.

Example 9 Other OMS Structures

SO₂ absorption capacities of other manganese oxides with tunnelstructure, including Todorokite-type magnesium manganese oxide withchannels of 3×3 MnO₆ units, sodium manganese oxide with channels of 2×4MnO₆ units, sodium manganese oxide with channels of 2×3 MnO₆ units, andpyrolusite manganese oxide with 1×1 MnO₆ units, were studied.

Pyrolusite, MnO₂ 1×1, was obtained from Stream Chemicals. Theas-received chemical was ball-milled for 1 hr to get ˜1 μm particlesbefore SO₂ absorption test. One todorokite material, OMS-1, was providedby Engelhard Corporation. Other tunnel-structured manganese oxides wereprepared in the lab using the methods described in the publishedliteratures.

Birnessite was used as precursor for the synthesis of channel-structuredmanganese oxides. Birnessite-type layered manganese oxides were preparedusing the methods used by Golden, et al. (5) and (6). A typicalsynthesis was carried out as: 250 ml 6.4 M NaOH solution was mixed with200 ml 0.5M MnSO₄ at room temperature. Oxygen was immediately bubbledthrough a glass frit at a rate of 4 L/min. After 4.5 h the oxygenationwas stopped and the precipitate was filtered out and washed withdeionized water 4 times, and then dried in air at 100° C. About 13 ggrey color birnessite product was obtained.

Two sodium manganese oxides with channels of 2×3 MnO₆ units (Na 2×3 A &B) were prepared by directly calcination of birnessite in air for 12 hat 500° C., and 650° C., respectively. (7).

Todorokite (magnesium manganese oxide with channels of 3×3 MnO₆ units)was prepared as described in (5) and (8): ˜3 g birnessite was added in100 ml 1M MgCl₂ solution, the mixture was shaken overnight at roomtemperature for Mg²⁺ ion exchange for Na⁺. The slurry was washed fourtimes with deionized water. Then, Mg²⁺-birnessite, together with 25 mlH₂O, was autoclaved at 150° C. for 48 hr. After washing with D.I water 3times, the product was dried in air at 100° C. About 2.0 gtodorokite-type tunnel structure manganese oxide (Mg 3×3) was obtained.

Sodium manganese oxide with channels of 2×4 MnO₆ units was preparedusing method developed by Xia et al. (9). About 5 g birnessite, togetherwith 25 ml 2.5M NaCl solution, was autoclaved at 210° C. for 48 hr.After washing with D.I water 3 times, the product was dried in air at100° C. About 4.4 g black color product, Na 2×4, was obtained.

The dried materials were sieved to provide 40–80 mesh particles for theSO₂ absorption test, which was carried out in a temperature controlledreactor with an the SCD analytical system. All these materials weretested under the same conditions (0.5 gram 40–80 mesh absorbentparticles, 100 sccm feed flow of 250 ppm SO₂ in 82% N₂, 18% O₂) at atemperature of 325° C. (See Table 5). Before each SO₂ absorptionmeasurement, the absorbent material was heated at 500° C. for 2 h inflowing air to remove residual moisture. To characterize the structurechange before and after SO₂ absorption, powder X-ray diffraction pattern(XRD) was collected on some of the tested materials.

TABLE 5 SO₂ absorption tests of other OMS materials Absorbent TestConditions* MnO₂ 1 × 1 0.5 g 40–80 mesh absorbent, 325° C., 100 sccmpyrolusite from Strem Chemicals, flow of 250 ppm SO₂, 82% N₂, 18% O₂,with 1 × 1 tunnels GHSV = 18K hr⁻¹ Na 2 × 3 A 0.5 g 40–80 meshabsorbent, 325° C., 100 sccm Na₂Mn₅O₁₀, with 2 × 3 tunnels, flow of 250ppm SO₂, 82% N₂, 18% O₂, calcined at 500° C. for 12 h GHSV = 3.4K hr⁻¹Na 2 × 3 B 0.5 g 40–80 mesh absorbent, 325° C., 100 sccm Na₂Mn₅O₁₀, with2 × 3 tunnels, flow of 250 ppm SO₂, 82% N₂, 18% O₂, calcined at 650° C.for 12 h GHSV = 5.1K hr⁻¹ Na 2 × 3 A 0.5 g 40–80 mesh absorbent, 325°C., 100 sccm Na₂Mn₅O₁₀, with 2 × 3 tunnels, flow of 250 ppm SO₂, 82% N₂,18% O₂, calcined at 500° C. for 12 h GHSV = 11K hr⁻¹ Na 2 × 4 0.5 g40–80 mesh absorbent, 325° C., 100 sccm sodium manganese oxide with 2 ×4 flow of 250 ppm SO₂, 82% N₂, 18% O₂, tunnels GHSV = 11K hr⁻¹ OMS-1 0.5g 40–80 mesh absorbent, 325° C., 100 sccm todorokite, with 3 × 3tunnels, flow of 250 ppm SO₂, 82% N₂, 18% O₂, provided by EngelhardCorporation GHSV = 11K hr⁻¹ Mg 3 × 3 0.5 g 40–80 mesh absorbent, 325°C., 100 sccm Todorokite, with 3 × 3 tunnels flow of 250 ppm SO₂, 82% N₂,18% O₂, Synthesized GHSV = 2.7 hr⁻¹ *Before each SO₂ absorptionmeasurement, the absorbent material was heated at 500° C. for 2 h in 100sccm air.

Example 10 Other OMS Structures Results and Comparison

The measured breakthrough capacities at selected gas flow rates for thematerials prepared in Example 9 are given in Table 6 along withexemplary capacities for the 2×2 structure of Example 1.

TABLE 6 SO₂ absorption capacity of OMS absorbents GHSV, Break throughMaterial tested hr⁻¹ capacity, wt % MnO₂, from Strem Chemicals, 1 × 1 18K <0.1 tunnels of MnO₆ units Na₂Mn₅O₁₀, 2 × 3 tunnels of MnO₆ units,3.4K 57.5 calcined at 500° C. for 12 h (A) Na₂Mn₅O₁₀, 2 × 3 tunnels ofMnO₆ units, 5.1K 31 calcined at 650° C. for 12 h (B) Na₂Mn₅O₁₀, 2 × 3tunnels of MnO₆ units,  11K 12 calcined at 500° C. for 12 h (A) SodiumManganese Oxide, 2 × 4 tunnels of  11K 33 MnO₆ units MgMn₂O₄ fromtodorokite (3 × 3) provided  11K 1.5 by Engelhard Corporation MgMn₂O₄from the synthesized todorokite 2.7K 53 (3 × 3) Cryptomelane, 2 × 2tunnels of MnO₆ units   8K 62 Cryptomelane, 2 × 2 tunnels of MnO₆ units 30K 42

Save two of the materials, the SO₂ breakthrough absorption capacitiesfor these materials are generally much higher than those of conventionalSO_(x) absorbents (normally less than 5 wt %), establishing theirusefulness as SO_(x) absorbents pursuant to the present invention.

The 1×1 MnO₂ from Strem Chemicals was confirmed by XRD to be wellcrystallized pyrolusite, which consists of 1×1 MnO₆ tunnels. Aftercalcination at 500° C. for 2 h in air, the structure remained stable.However the material exhibits poor absorption capacity, with abreakthrough capacity of about 0.1% and a maximum absorption capacityless than 3%.

The birnessite synthesized in this work was calcined in air either at500° C. for 12 h (A), or at 650° C. for 12 h (B). In either case, sodiummanganese oxide, Na₂Mn₅O₁₀, forms, the basic structure which consists ofMnO₆ octahedra joined at edges to form a 2×3 tunnel structure. (7) FIG.8 shows the SO₂ absorption performance of both the A and B formulationsof this microporous manganese oxide under different GHSV. Similar to the2×2 cryptomelane materials, this microporous manganese oxide also hasvery high SO₂ absorption capacity. At 3444 hr⁻¹ GHSV, the total SO₂absorbed is ˜70 wt %. At higher GHSV, the SO₂ absorption performancedecreases, indicating the reaction is controlled by the mass diffusionof SO₂ through the absorbent. After SO₂ absorption, MnSO₄ forms.

The XRD pattern of the sodium manganese oxide synthesized byhydrothermal reaction in this work matched very well with the XRDpattern of the sodium manganese oxide material with 2×4 tunnel structurepublished by Xia et al. (10) Likewise, as expected, after calcination at500° C. for 12 h in air, the 2×4 tunnel structure remains unchanged.FIG. 9 gives the SO₂ absorption test result at 10755 hr⁻¹ GHSV. Thismaterial also has high SO₂ absorption capacity.

While the todorokite magnesium manganese oxide prepared by hydrothermalreaction in this work was not well crystallized, the todorokitestructure could still be identified in the XRD. However, aftercalcination at 500° C. for 2 h in air, the 3×3 tunnel structure changedmostly to MgMn₂O₄. This structure change was evident in the MgMn₂O₄provided by Engelhard Corporation as well. While the todorokitematerials both exhibited relative high absorption capacities, thisinstability at moderately high temperatures is a relative disadvantagefor most applications. While the could be used as absorbents, mostconventional implementations require stability above the absorbents at500° C. or sometimes higher.

Closure

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character. Only certain embodimentshave been shown and described, and all changes, equivalents, andmodifications that come within the spirit of the invention describedherein are desired to be protected. Any experiments, experimentalexamples, or experimental results provided herein are intended to beillustrative of the present invention and should not be consideredlimiting or restrictive with regard to the invention scope. Further, anytheory, mechanism of operation, proof, or finding stated herein is meantto further enhance understanding of the present invention and is notintended to limit the present invention in any way to such theory,mechanism of operation, proof, or finding. Thus, the specifics of thisdescription and the attached drawings should not be interpreted to limitthe scope of this invention to the specifics thereof. Rather, the scopeof this invention should be evaluated with reference to the claimsappended hereto. In reading the claims it is intended that when wordssuch as “a”, “an”, “at least one”, and “at least a portion” are usedthere is no intention to limit the claims to only one item unlessspecifically stated to the contrary in the claims. Further, when thelanguage “at least a portion” and/or “a portion” is used, the claims mayinclude a portion and/or the entire items unless specifically stated tothe contrary. Likewise, where the term “input” or “output” is used inconnection with a fluid processing unit, it should be understood tocomprehend singular or plural and one or more fluid channels asappropriate in the context. Finally, all publications, patents, andpatent applications cited in this specification are herein incorporatedby reference to the extent not inconsistent with the present disclosureas if each were specifically and individually indicated to beincorporated by reference and set forth in its entirety herein,including the following scientific publications referenced in thespecification above:

(1) X. Chen, Y. F. Shen, S. L. Suib, and C. L. O'Young,“Characterization of Manganese Oxide Octahedral Molecular Sieve(M-OMS-2) Materials with Different Metal Cation Dopants”, Chem. Mater.2002, 14, 940–948.

(2) G. G. Xia, Y. G. Yin, W. S. Willis, J. Y. Wang, and S. L. Suib,“Efficient Stable Catalysts for Low Temperature Carbon MonoxideOxidation”, J. Catal. 1999, 185, 91–105.

(3) R. N. DeGuzman, Y. F. Shen, E. J. Neth, S. L. Suib, C. K. O'Young,S. Levine, and J. M. Newsam, “Synthesis and Characterization ofOctahedral Molecular Sieves (OMS-2) Having the Hollandite Structure”,Chem. Mater. 1994, 6 815–821.

(4) S. L. Suib, C-L O'Young, “Synthesis of Porous Materials”, M. L.Occelli, H. Kessler, eds, M. Dekker, Inc., p. 215, 1997.

(5) D. C. Golden, C. C. Chen, and J. B. Dixon, “Synthesis ofTodorokite”, Science, 1986, 231, 717–719.

(6) D. C. Golden, C. C. Chen, and J. B. Dixon, “Transformation ofbirnessite to buserite, todorokite, and manganite under mildhydrothermal treatment”, Clays Clay Miner. 1987, 35, 271–280.

(7) J. P. Parant, R. Olazcuaga, M. Devalette, C. Fouassier, and P.Hagenmuller, “Sur Quelques Nouvelles Phases de Formule Na_(x)MnO₂(x≦1)”, J. Solid State Chem. 1971, 3, 1–11.

(8) Y. F. Shen, R. P. Zerger, R. N. DeGuzman, S. L. Suib, L. McCurdy, D.I. Potter, and C. L. O'Young, “Manganese oxide octahedral molecularsieves: preparation, characterization, and applications”, Science, 1993,260, 511–515

(9) G. G. Xia, W. Tong, E. N. Tolentino, N. G. Duan, S. L. Brock, J. Y.Wang, S. L. Suib, and T. Ressler, “Synthesis and characterization ofnanofibrous sodium manganese oxide with a 2×4 tunnel structure”, Chem.Mater, 2001, 13, 1585–1592.

1. A method for emissions control comprising: at a location upstreamfrom a particulate filter or NOx trap, removing a substantial quantityof a sulfur oxide from a combustion exhaust by contacting the exhaustwith a quantity of a sorbent comprising a manganese-based octahedralmolecular sieve (Mn—OMS).
 2. The method of claim 1 wherein the Mn—OMShas the formula X_(a)Mn₈O₁₆ wherein X is selected from the groupconsisting of alkali metals and alkaline earth metals and a is between0.5 and 1.5.
 3. The method of claim 2 wherein X is potassium.
 4. Themethod of claim 1 wherein the contacting is at a temperature of at leastabout 100° C.
 5. The method of claim 4 wherein the Mn—OMS has a sulfurdioxide absorption capacity of at least about 40% by weight at thecontacting temperature.
 6. The method of claim 1 wherein the exhaust isfrom a motor vehicle.
 7. The method of claim 6 wherein the exhaust is adiesel engine exhaust.
 8. The method of claim 1 wherein MnSO₄ is formedby the contacting, the method further comprising regenerating thesorbent after the contacting by reacting the MnSO₄ with KOH and O₂. 9.The method of claim 1 wherein the contacting removes at least about 90%of the sulfur oxide from the exhaust.
 10. The method of claim 1 whereinthe Mn—OMS is formed from MO₆ octahedra connected together such that thestructure generates micropores in the form of channels of A×B octahedra,wherein A and B are integers from 2 to
 4. 11. The method of claim 10wherein more than 50% of the M elements by mole are manganese.
 12. Themethod of claim 11 wherein a cation selected from H⁺, NH₄ ⁺, Li⁺, Na⁺,Ag⁺, K⁺, Rb⁺, Tl⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ra²⁺, Cu²⁺, Pb²⁺ islocated is the channels.
 13. The method of claim 12 wherein the cationis K⁺.
 14. The method of claim 10 wherein up to 5% of the M elements aredopant selected from copper, chromium, iron, nickel, cobalt, zinc,aluminum, gallium, titanium, tin, lead, antimony, indium, silicon,germanium, titanium and combinations thereof.
 15. The method of claim 14wherein at least 1% of the M elements are the dopant.
 16. A system foremissions control comprising: a source of a combustion exhaust stream;and first and second emissions control devices receiving the exhauststream; wherein the first emission control device is upstream from thesecond emission control device; wherein the first emission controldevice contains sorbent for removing a substantial quantity of sulfurdioxide from the exhaust stream; wherein the sorbent comprises amanganese-based octahedral molecular sieve (Mn—OMS).
 17. The system ofclaim 16 wherein the second emission control device is a particulatefilter or NOx trap.
 18. The system of claim 17 wherein the source of acombustion exhaust stream is the engine of a vehicle.
 19. The system ofclaim 18 wherein the engine is a diesel engine.
 20. The system of claim19 wherein the sorbent has a sulfur dioxide absorption capacity greaterthan about 40% by weight at a temperature greater than 200° C.
 21. Thesystem of claim 16 wherein the sorbent has an Mn—OMS structure is a 2×2structure.
 22. The system of claim 16 wherein the first emission controldevice includes a quantity of the sorbent sufficient to remove at leastabout 90% of the sulfur dioxide in the exhaust stream over at least 24hours of normal operation of the source of the combustion exhauststream.
 23. The system of claim 22 wherein the sorbent has a 2×2structure.
 24. A method for removing sulfur dioxide from a gascomprising: removing at least about 95% of the sulfur dioxide in agaseous stream by passing the gaseous stream through a sorbent bed,wherein the sorbent bed includes a manganese-based octahedral molecularsieve (Mn—OMS) on a support.
 25. The method of claim 24 wherein thegaseous stream is less than 1 molar percent oxygen.
 26. The method ofclaim 24 wherein the gaseous stream is substantially devoid of oxygen.27. The method of claim 24 wherein the gaseous stream is a combustionexhaust.
 28. A low emission motor vehicle comprising: a combustionengine for powering the vehicle; a first emission control devicecontaining a sorbent and receiving an exhaust of the engine; and asecond emission control device downstream from the first emissioncontrol device for removing particulates and/or NO_(x) from the exhaustwhich has passed through the first emission control device; wherein thesorbent includes a quantity of a manganese-based octahedral molecularsieve (Mn—OMS) for substantially reducing the amount of SO₂ that wouldotherwise enter the second emission control device.
 29. The motorvehicle of claim 28 wherein the engine is a diesel engine.
 30. The motorvehicle of claim 28 wherein the first emission control device includes ahousing having a window for determining the color of the sorbent. 31.The motor vehicle of claim 28 wherein the sorbent has a 2×2 structure.32. A method comprising: substantially reducing the levels of sulfuroxides in an exhaust gas by contacting the exhaust gas with a materialselected from materials with structure type OMS 2×2, OMS 2×3 and OMS3×3, formed from MO₆ octahedra connected together such that thestructure generates micropores in the form of channels, said octahedracomprising at least one element (M) selected from elements from groupsIIIB, IVB, VB, VIIB, VIIB, VIII, IB, IIB and IIIA of the periodic tableand germanium wherein at least a major portion of element (M) ismanganese, said material further comprising at least one element (B)selected from the group formed by the alkaline elements, thealkaline-earth elements, the rare earth elements, the transition metalsand elements from groups IIIA, IVA of the periodic table.
 33. The methodof claim 32 wherein the average valence of the metals (M) is betweenabout +3.5 and +4.
 34. The method of claim 32 wherein at least a majorportion of said element (B) is selected from potassium, sodium,strontium, copper, zinc, magnesium, rubidium and calcium and mixtures ofat least two of said elements.
 35. The method of claim 34 wherein atleast 75% of the element M is manganese.
 36. An emissions control devicecomprising: a housing defining an inlet and an outlet; wherein aquantity of a sulfur oxide sorbent is contained in the housing; whereinthe sorbent comprises a manganese-based octahedral molecular sieve(Mn—OMS); wherein the housing includes a window for monitoring the colorof the sorbent.
 37. The device of claim 36 wherein the inlet is coupledto an exhaust stream from the internal combustion engine of a motorvehicle.
 38. A method comprising: removing sulfur oxides from theexhaust of an internal combustion engine with an emissions controldevice according to claim 36; and removing the emissions control devicefrom the exhaust based on color change of the sorbent.